CHANNELING ALIGNMENT FOR EPITAXIAL LIGHT-MASS FILM ON HEAVY-MASS SUBSTRATE

A simple procedure for obtaining a background-free backscattering spectrum of a light-mass film on a heavy-mass substrate by a normal incidence/grazing exit geometry has been described. Using this method such films can be aligned rapidly and accurately, and the impurity or defect information on the films can be obtained without need for realignment. Example is given from MeV Li-3+ analysis of a deposited film of Si on a single crystal substrate of yttria-stabilized, cubic zirconia.

STUDY ON PREPARATION OF GAN AND COSI2 EPITAXIAL-FILMS BY MASS ANALYZED LOW-ENERGY DUAL ION-BEAM EPITAXY

The Mass Analyzed Low Energy Dual Ion Beam Epitaxy (MALE-DIBE) system has been designed and constructed in our laboratory. We believe that the system, which was installed and came into full operation in 1988, is the first facility of this kind. With our system we have carried out studies, for the first time, on compound synthesis of GaN and CoSi2 epitaxial thin films. RHEED and AES results show that GaN films, which were deposited on Si and sapphire substrates, are monocrystalline and of good stoichiometry. To our knowledge, GaN film heteroepitaxially grown on Si. which is more lattice-mismatched than GaN on sapphire, has not been reported before by other authors. RBS and TEM investigations indicated a rather good crystallinity of CoSi2 with a distinct interface between CoSi2 and the Si substrate. The channelling minimum yield chi(min) from the Co profile is approximately 4%. The results showed that the DIBE system with simultaneous arrival of two beams at the target is particularly useful in the formation of novel compounds at a relatively low substrate temperature.

CONSTRUCTION AND APPLICATIONS OF A DUAL MASS-SELECTED LOW-ENERGY ION-BEAM SYSTEM

A direct ion beam deposition system designed for heteroepitaxy at a low substrate temperature and for the growth of metastable compounds has been constructed and tested. The system consists of two mass-resolved low-energy ion beams which merge at the target with an incident energy range 50-25 000 eV. Each ion beam uses a Freeman ion source for ion production and a magnetic sector for mass filtering. While a magnetic quadrupole lens is used in one beam for ion optics, an electrostatic quadrupole lens focuses the other beam. Both focusing approaches provide a current density more than 100-mu-A/cm2, although the magnetic quadrupole gives a better performance for ion energies below 200 eV. The typical current of each beam reaches more than 0.3 mA at 100 eV, with a ribbon beam of about 0.3-0.5 x 2 cm2. The target is housed in an ultrahigh vacuum chamber with a base pressure of 1 x 10(-7) Pa and a typical pressure of 5 x 10(-6) Pa when a noncondensable beam like argon is brought into the chamber. During deposition, the target can be heated to 800-degrees-C and scanned mechanically with an electronic scanning control unit. The dual beam system has been used to grow GaN using a Ga+ and a N+ beam, and to study the oxygen and hydrogen ion beam bombardment effects during carbon ion beam deposition. The results showed that the simultaneous arrival of two beams at the target is particularly useful in compound formation and in elucidation of growth mechanisms.

EFFECTIVE-MASS THEORY FOR SUPERLATTICES GROWN ON (11N)-ORIENTED SUBSTRATES

An effective-mass formulation for superlattices grown on (11N)-oriented substrates is given. It is found that, for GaAs/AlxGa1-xAs superlattices, the hole subband structure and related properties are sensitive to the orientation because of the large anisotropy of the valence band. The energy-level positions for the heavy hole and the optical transition matrix elements for the light hole apparently change with orientation. The heavy- and light-hole energy levels at k parallel-to = 0 can be calculated separately by taking the classical effective mass in the growth direction. Under a uniaxial stress along the growth direction, the energy levels of the heavy and light holes shift down and up, respectively; at a critical stress, the first heavy- and light-hole energy levels cross over. The energy shifts caused by the uniaxial stress are largest for the (111) case and smallest for the (001) case. The optical transition matrix elements change substantially after the crossover of the first heavy- and light-hole energy has occurred.

Effective-mass theory for InAs/GaAs strained superlattices

By using the recently developed exact effective-mass envelope-function theory, the electronic structures of InAs/GaAs strained superlattices grown on GaAs (100) oriented substrates are studied. The electron and hole subband structures, distribution of electrons and holes along the growth direction, optical transition matrix elements, exciton states, and absorption spectra are calculated. In our calculations, the effects due to the different effective masses of electrons and holes in different materials and the strain are included. Our theoretical results are in agreement with the available experimental data.

Cyclotron-resonance mass of two-dimensional electrons in GaN/AlxGa1-xN heterostructures

The electron cyclotron-resonance (CR) mass of quasi-two-dimensional electrons in GaN/AlxGa1-xN heterostructures is studied theoretically. The correction to the CR mass due to electron-phonon interaction is investigated, taking into account band nonparabolicity, the occupation effect, and the screening of the electron-phonon coupling. The dependence of the CR mass on the electron density and on the magnetic field strength is displayed in detail, and the calculated CR mass agrees well with a recent experiment. We found that the effective electron-phonon coupling strength in GaN heterostructures is reduced below the bulk value.

Effective-mass theory for InAs/GaAs strained coupled quantum dots

In the framework of effective-mass envelope-function theory, the optical transitions of InAs/GaAs strained coupled quantum dots grown on GaAs (100) oriented substrates are studied. At the Gamma point, the electron and hole energy levels, the distribution of electron and hole wave functions along the growth and parallel directions, the optical transition-matrix elements, the exciton states, and absorption spectra are calculated. In calculations, the effects due to the different effective masses of electrons and holes in different materials are included. Our theoretical results are in good agreement with the available experimental data.

Electro-mass multi odor sensor

For an olfactory sensor or electronic nose the task is not only to detect the object concentration, but also to recognize it. It is well known that all the elements can be identified by their charge to mass ratio e+/m. We tried to use this principle for molecular recognition. Two kinds of sensors are used simultaneously in testing. One is Quartz Crystal Microbalance (QCM) for detecting the change in mass, the other is Interdigital Electrode (IE) for detecting the change in conduction. In this paper the principle and the feasibility of this method are reported. The preliminary results on the recognition of alcohols are presented. The multisensor can be used as an instrument for research on material properties and kinetic process as well.

Electron effective mass and resonant polaron effect in CdTe/CdMgTe quantum wells

Cyclotron resonance in CdTe/CdMgTe quantum wells (QWs) was studied. Due to the polaron effect the zero-field effective mass is strongly influenced by the QW width. The experimental data have been described theoretically by taking into account electron-phonon coupling and the nonparabolicity of the conduction band. The subband structure was calculated self-consistently. The best fit was obtained for an electron-phonon coupling constant alpha = 0.3 and bare electron mass of m(b) = 0.092m(0).

Electro-mass olfactory multi-sensor (EMMS)

For an olfactory sensor or electronic nose, the task is not only to detect the object concentration, but also to recognize it. It is well known that all the elements can be identified by their charge to mass ratio e(+)/m. We tried to imitate this principle for molecular recognition. Two kinds of sensors are used simultaneously in testing. One is quartz crystal microbalance (QCM) for detecting the change in mass, the other is interdigital electrode (IE) for detecting the change in conduction, as an electro-mass multi-sensor (EMMS). in this paper, the principle and the feasibility of this method are discussed. The preliminary results on the recognition of alcohol by EMMS coated with lipids are presented. Meanwhile, the multi-sensor can also be used as an instrument for research on some physico-chemistry problems. The change in conduction of coated membrane caused by one absorbed molecule is reported. It is found that when a QCM is coated with membrane, it still obeys the relationship Delta F (frequency change of QCM) = K Delta m (mass change of absorbed substance) and the proportional coefficient, K, depends not only on quartz properties but also on membrane characteristics as well. (C) 2000 Elsevier Science S.A. All rights reserved.

金黄色葡萄球菌肠毒素C2的结构特征与生物学功能研究

通过PCR技术对Staphylococcal enterotoxin C2(SEC2)的氨基端和羧基端部位进行删除突变。研究结果显示删除羧基端77个氨基酸对SEC2超抗原和致热活性没有显著影响，但更进一步的删除导致SEC2活性显著降低。而少量氨基端残基删除则可导致SEC2超抗原活性显著降低，说明SEC2超抗原活性主要集中于其氨基端部位。以上研究结果也表明SEC2分子的完整性并不是其超抗原活性所必需的。 对二硫环及Zn结合位点配位组氨酸残基在SEC2分子中的生物学功能进行研究。以定点突变技术构建SEC2二硫环突变蛋白，从而干扰二硫键的形成。研究结果表明干扰二硫环形成对SEC2的超抗原、催吐和致热等活性均有显著影响，说明二硫环在维持SEC2生物学活性方面具有重要作用。此外，针对Zn结合位点配位组氨酸残基的研究结果显示突变118位和122位组氨酸后对SEC2的超抗原活性没有有显著影响，而47位组氨酸的突变导致SEC2分子的超抗原活性显著降低。动物实验结果显示分别突变118位和122位组氨酸残基后均导致SEC2分子催吐及热源活性显著降低，而47位组氨酸突变仍具有较高的催吐和致热活性。说明Zn结合位点配位组氨酸残基在SEC2的催吐、致热和超抗原活性中扮演了重要的作用，并进一步揭示肠毒素诱导的超抗原和致热活性之间没有明显的相关性。 以Leu及Glu分别替代20和22位氨基酸残基后SEC2超抗原活性有显著提高，并将具有减毒作用的组氨酸位点突变引入上述活性增强的SEC2突变，从而构建减毒突变蛋白。结果显示引入118和122位组氨酸双突变后突变蛋白保留了与SEC2相当的超抗原和抗肿瘤活性，但与SEC2相比致热活性有显著下降，为减毒SEC2超抗原药物的开发提供了可能性。 为探索金黄色葡萄球菌的体内基因突变方法，本研究对金黄色葡萄球菌基因组中的α-溶血毒素基因进行敲除。首先构建同源重组质粒pMHL-α，经金黄色葡萄球菌RN4220修饰后再通过原生质体转入金黄色葡萄球菌SM-01。含重组质粒pMHL-α的金黄色葡萄球菌SM-01在42℃诱导条件下培养多代，最终筛选出α-溶血毒素基因缺失菌株。经序列分析和血平板溶血实验结果证实最终获得产SEB金黄色葡萄球菌α-HL缺失菌株。为构建产减毒肠毒素的金黄色葡萄球菌基因工程菌株提供了一定的理论基础和方法。